I Want my a4 b6 1.8T to have abit more grunt and i want to change the exhaust tips but a friend told me to remove the boxjust before the tip and replace it with a straight pipe,he did it with his astra and it sounds nice but I am not sure if it is going to effect anything
any advice would be much appriciated:)

best way is to remove the rear boxes and go for a short drive, of course this could be illegal so at your own risk, but then before chopping and wasting money you can get a idea.

it will sound nice but you may find it drones on long trips. and have you buying a decent set of boxes from reputable company.

best way, full system, also try youtube with a decent set of ear/headphones to get a idea